Windom antennas - down to earth
 

According to my ARRL Antenna Handbook, the Windom Antenna was described
by Loren Windom in QST in 1929.

The design is a horizontal half wave (ie single band) fed by a vertical
single wire feedline attached just off centre (~14%). Explanations go
that this approximately matches the feedline Zo (which is quite high)
with the horizontal wire. It is single wire (ie ultimately unbalanced)
feedline and therefore radiates. The Antenna is fed between the source
end of the feedline and ground, and the load impedance should be
somewhere in the many hundreds of ohms. The feedline carries an
appreciable net current.

More recently, the Off Centre Fed (OCF) Dipole design emerged,
principally as a multi-band antenna. The OCF Dipole is a horizontal wire
with a coaxial feed and coupling transformer (often called a balun)
attached offset from the centre of the dipole. The feedpoint excursions
at a half wave length and harmonic frequencies are much lower than centre
feeding, and may be operated as a multiband antenna with reasonable
efficiency, though it probably really needs an ATU at the tx end of the
coax. The OCF dipole feedline does have current flowing on the outer of
the outer conductor, at least as a result of the assymetric coupling to
the dipole legs, and to some extent because the ineffectiveness of
practical coupling transformers to isolate the feedline ends from the
differing voltages on each dipole leg. The feedline carries an
appreciable net current.

Then along came the Carolina Windom, which appears to be a OCF dipole
with a proprietary (ie secret, undescribed) coupling transformer, a
vertical coax section (feedline and radiator) and a proprietary (remember
the meaning) "isolator" located at a given distance along the coax to
prevent the current flowing on the outer of the outer of the coax from
flowing further towards the transmitter. The isolater would appear to be
a ferrite choke and it would introduce a series impedance (reactance and
resistance) to current on the coax, so influencing the establishment of
the standing wave pattern on the outer of the outer of the coax. You
might naively think that this isolator prevents current flowing into the
shack, but that is unlikely.

In all these cases, there is an expectation that the feedline carries a
net radiating current, and it seems to me, that if you don't want to
bring that into the shack, you need to design an appropriate solution.

In the case of the true Windom, it seems the easiest solution is to end
the single wire feedline outside the shack and place a matching unit
connecting to ground and the single wire feedline at that point, and
transforming the load to something suitable to coax or balanced feedline
to the shack.

In the case of the OCF Dipole and the Carolina Windom, shunting the
current on the outer of the outer to ground outside the shack is a
potential solution. Series chokes might help, but the magnitude of the
choking impedance is limited, and their effectiveness could be improved
greatly by a low impedance shunt to ground.

Comments?

Owen

PS: In todays paranoid world where rules in many jurisdictions restrict
the maximum permitted exposure to electromagnetic radiation, antennas
such as these with radiating elements that are close to areas accessible
by people are a safety challenge.

Windom antennas - down to earth
 

Owen Duffy wrote:
According to my ARRL Antenna Handbook, the Windom Antenna was described
by Loren Windom in QST in 1929.

The design is a horizontal half wave (ie single band) fed by a vertical
single wire feedline attached just off centre (~14%). Explanations go
that this approximately matches the feedline Zo (which is quite high)
with the horizontal wire. It is single wire (ie ultimately unbalanced)
feedline and therefore radiates. The Antenna is fed between the source
end of the feedline and ground, and the load impedance should be
somewhere in the many hundreds of ohms. The feedline carries an
appreciable net current.

More recently, the Off Centre Fed (OCF) Dipole design emerged,
...

EXCELLENT page on OCF construction:

http://www.radioelectronicschool.net.../ocfdipole.pdf

Will take a bit to load up in adobe reader (is a BIG file) on slow 56K
dialup connection.

JS
--
http://assemblywizard.tekcities.com

Windom antennas - down to earth
 

John Smith I wrote in news:esfki4$k29$1
@nnrp.linuxfan.it:

Owen Duffy wrote:
EXCELLENT page on OCF construction:

http://www.radioelectronicschool.net.../ocfdipole.pdf


Yes, a good article. But Ron doesn't really address the common mode current
issue, and routing common mode current into the shack.

Seems like an excuse to give him a ring, we haven't talked in a while.

Owen

Windom antennas - down to earth
 

Owen Duffy wrote:

...
Yes, a good article. But Ron doesn't really address the common mode current
issue, and routing common mode current into the shack.

Seems like an excuse to give him a ring, we haven't talked in a while.

Owen

Yes, for that any good page on voltage\current baluns or hybrid baluns
will bring one up to speed. For example, a page which provides more
than you want to know:

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12661

--
http://assemblywizard.tekcities.com

Windom antennas - down to earth
 

John Smith I wrote in
:

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12661

Hmmm, content hijackers.

Windom antennas - down to earth
 

I tried a real Windom on 20 meters. Used the ARRL antenna book
formula. Ran a single wire feedline to the random wire terminal of an
old Murch transmatch. Had a 1/4 wave elevated counterpoise attached to
the ground terminal of the tuner. I couldn't match the thing to my
little QRP rig to save my soul. Lots of RF and buzzing sounds.

Ended up turning it into a standard dipole.

bob
k5qwg

On Sun, 04 Mar 2007 22:07:30 GMT, Owen Duffy wrote:


According to my ARRL Antenna Handbook, the Windom Antenna was described
by Loren Windom in QST in 1929.

The design is a horizontal half wave (ie single band) fed by a vertical
single wire feedline attached just off centre (~14%). Explanations go
that this approximately matches the feedline Zo (which is quite high)
with the horizontal wire. It is single wire (ie ultimately unbalanced)
feedline and therefore radiates. The Antenna is fed between the source
end of the feedline and ground, and the load impedance should be
somewhere in the many hundreds of ohms. The feedline carries an
appreciable net current.

More recently, the Off Centre Fed (OCF) Dipole design emerged,
principally as a multi-band antenna. The OCF Dipole is a horizontal wire
with a coaxial feed and coupling transformer (often called a balun)
attached offset from the centre of the dipole. The feedpoint excursions
at a half wave length and harmonic frequencies are much lower than centre
feeding, and may be operated as a multiband antenna with reasonable
efficiency, though it probably really needs an ATU at the tx end of the
coax. The OCF dipole feedline does have current flowing on the outer of
the outer conductor, at least as a result of the assymetric coupling to
the dipole legs, and to some extent because the ineffectiveness of
practical coupling transformers to isolate the feedline ends from the
differing voltages on each dipole leg. The feedline carries an
appreciable net current.

Then along came the Carolina Windom, which appears to be a OCF dipole
with a proprietary (ie secret, undescribed) coupling transformer, a
vertical coax section (feedline and radiator) and a proprietary (remember
the meaning) "isolator" located at a given distance along the coax to
prevent the current flowing on the outer of the outer of the coax from
flowing further towards the transmitter. The isolater would appear to be
a ferrite choke and it would introduce a series impedance (reactance and
resistance) to current on the coax, so influencing the establishment of
the standing wave pattern on the outer of the outer of the coax. You
might naively think that this isolator prevents current flowing into the
shack, but that is unlikely.

In all these cases, there is an expectation that the feedline carries a
net radiating current, and it seems to me, that if you don't want to
bring that into the shack, you need to design an appropriate solution.

In the case of the true Windom, it seems the easiest solution is to end
the single wire feedline outside the shack and place a matching unit
connecting to ground and the single wire feedline at that point, and
transforming the load to something suitable to coax or balanced feedline
to the shack.

In the case of the OCF Dipole and the Carolina Windom, shunting the
current on the outer of the outer to ground outside the shack is a
potential solution. Series chokes might help, but the magnitude of the
choking impedance is limited, and their effectiveness could be improved
greatly by a low impedance shunt to ground.

Comments?

Owen

PS: In todays paranoid world where rules in many jurisdictions restrict
the maximum permitted exposure to electromagnetic radiation, antennas
such as these with radiating elements that are close to areas accessible
by people are a safety challenge.

Windom antennas - down to earth
 

John Smith I wrote:
Owen Duffy wrote:

...
Yes, a good article. But Ron doesn't really address the common mode
current issue, and routing common mode current into the shack.

Seems like an excuse to give him a ring, we haven't talked in a while.

Owen

Yes, for that any good page on voltage\current baluns or hybrid baluns
will bring one up to speed. For example, a page which provides more
than you want to know:

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12661


Practical example of winding a guanella type balun ... (good detail in
picture depicting green/white wires)

http://www.n0ss.net/qrp_4-1_guanella-type_balun.pdf

JS
--
http://assemblywizard.tekcities.com

Windom antennas - down to earth
 

On Mar 4, 4:07 pm, Owen Duffy wrote:
According to my ARRL Antenna Handbook, the Windom Antenna was described
by Loren Windom in QST in 1929.


I can't stand windoms myself... A poor design overall.. Bout par for
1929
technology..
I compared a dipole to one of those "carolina windoms" one time using
a
A/B switch.. It was ugly.. The dipole thrashed it. The windom setup
had
way too much loss. Mostly in the perverted mess of a feedline/tuner I
suspect.
But.... It doesn't bother me if anyone else wants to use them. IE:
field day,
etc. I like having an unfair advantage... :) All those windoms strung
across
the country on field day just help me get a better score...
Ughhhmm, maybe I should keep my mouth shut...
Yea.. Forget what I said. The windoms are great antennas.
A very efficient antenna system, and everyone should use one. If you
don't
use a windom at field day, you ain't really living.. Those funky fed
tuner/choke
G5RV's also have a special place in my heart.
Greatest antenna I've ever used.. :/ Grrrrr... I still remember
losing about
3 mm of tooth length due to excessive grinding when using a "funky
fed" G5RV
on 80m at field day a few years ago..
MK

Windom antennas - down to earth
 

John Smith I wrote in news:esg31b$n0a$1
@nnrp.linuxfan.it:

John Smith I wrote:
Practical example of winding a guanella type balun ... (good detail in
picture depicting green/white wires)

http://www.n0ss.net/qrp_4-1_guanella-type_balun.pdf

Notwithstanding all the focus on current baluns, a current balun does not
(by itself) prevent common mode current on an OCF Dipole feedline. The
feedline has assymetric mutual coupling to the dipole and can not be
expected in any configuration of significant length to be balanced wrt the
dipole over a wide frequency range.

Owen

Windom antennas - down to earth
 

Owen Duffy wrote:
John Smith I wrote in news:esg31b$n0a$1
@nnrp.linuxfan.it:

John Smith I wrote:
Practical example of winding a guanella type balun ... (good detail in
picture depicting green/white wires)

http://www.n0ss.net/qrp_4-1_guanella-type_balun.pdf

Notwithstanding all the focus on current baluns, a current balun does not
(by itself) prevent common mode current on an OCF Dipole feedline. The
feedline has assymetric mutual coupling to the dipole and can not be
expected in any configuration of significant length to be balanced wrt the
dipole over a wide frequency range.

Owen

Wide freq range?

Well, 40-50uh should provide 500 ohms impedance to common mode currents
on the outside of the braid of 50 ohm coax at ~2mhz, this is about the
minimum usable (in my humble opinion), although I have seen designs only
providing a 5:1 "common mode impedance to coax impedance", at lowest
freq, in common use (and especially when spanning wide bandwidth. And,
all figures used here are approx. and taken from my guess/memory/past
experiments--although I am sure they are in the ballpark)

At ~10 meters, the same inductance (same current balun) will present an
impedance of 7,000+ ohms to common mode currents (impedance increasing
with freq though the HF bands)

It is true, coupling to the coax, past the current balun can occur (and
probably most noticeable at low freqs/high power.) I seen a post by
Richard addressing that very problem, I believe, and he is using ferrite
beads along a section of coax leading away from the antenna, and
apparently in addition to some other current balun at the antenna. I
have seen others using an additional 1:1 current balun 1/4 wave away
from the antenna on the coax, or near their rig.

However, you will recognize that "magical" point where you realize
minimum/reduced/tolerable common mode currents as the coax can be moved
about without wild swings in SWR performance and you have a good match
over the freqs/bands which the antenna is designed for (given that you
don't have one end of the antenna close to a large metal building or
some other structure/object providing an object to couple to and set up
a LARGE imbalance.)

Now, the above is only VERY general. As even the
bifilar/trifilar/quadfiler windings and their spacings are of importance
to a properly designed and constructed balun and will affect its'
performance.

I make only VERY GENERAL statements in the construction of these baluns
and I don't wish to accept ANY responsibility in the results of someone
using them.

There are plentiful designs, many-many tried and true, available on the
net and I leave finding those as an exercise to anyone wishing to use them.

There is also abundant formulas/equations and examples given, on the
net, on how to design your own (material to use for rod/toroid, wire,
spacing, etc.)

And someday we will talk about the "rest of the story", those guys who
can hook a coathanger to their rig with a wet thread and realize good
characteristics!!! (well, almost grin)

Personally, I use the current balun whether it appears I need it or not,
I find the "insertion loss" tolerable. But, I agree, in a perfect world
there would be a better solution ...

JS
--
http://assemblywizard.tekcities.com